Firetube having thermal conducting passageways

ABSTRACT

A firetube is immersed in a fluid to be heated and transfers heat from hot gases flowing through the firetube to the fluid surrounding the firetube. The firetube has a plurality of thermally conductive passageways which extend through the firetube for increasing the surface area available for heat transfer. Fluid is conducted through the passageways by a thermosiphon effect resulting from a temperature differential in the vessel, the fluid below the firetube being cooler and denser than fluid above the heat exchanger.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application61/422,810, filed Dec. 14, 2010, and U.S. provisional application61/434,258, filed Jan. 19, 2011, the entirety of each of which isincorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the invention relate to indirect-fired and direct-firedheat exchangers and more particularly to firetubes installed in processvessels, the firetubes having enhanced surface area for heating processfluids.

BACKGROUND OF THE INVENTION

It is known to heat process fluids in a variety of vessels, such as ASMEcode process vessels, atmospheric bath heaters and tanks. Generally, aheat exchanger is fit within a vessel for heating fluids, such as thosecommonly handled in oilfield handling and refining operations.

In the oilfield, U-shaped “firetubes”, referred to as U-tube firetubesor U-tubes, are common heat exchangers for use in vessels containingfluids to be heated, such as heater-treaters, free water knock-outvessels, and in-line heaters and tanks. Traditionally the U-tubefiretube is made of round steel pipe. A burner supplies a flame and hotexhaust gases for circulation through the firetube from an inlet to anoutlet. Heat is conducted from the pipe walls to the fluid contained inthe vessel.

In a direct-fired vessel, heat is transferred through the firetube wallimmersed directly in a process fluid to be heated, the process fluidbeing contained in the vessel and in direct contact with the outside ofthe firetube. In an indirect-fired vessel, heat is transferred from thefiretube to an intermediate heat exchange fluid. A fluid-to-fluid heatexchanger contains the process fluid, the exchanger being immersed inthe heat exchange fluid.

Conventional U-tube firetubes have the burner mounted at the gas inletend of the firetube. A vent or exhaust stack is connected to the gasoutlet. Both the gas inlet and gas outlet are mounted in a common wallof the vessel. The U-tube exchanger is generally installed inside thevessel through an oval or obround shaped manway.

The ultimate objective in any fired heating system is to create thehighest thermal input possible for a given space. The thermal input isrelated in part to the surface area exposed to the hot exhaust gases onone side of the firetube wall and the fluid to be heated on the otherside. Use of round pipe to create the U-tube firetube results in a veryinefficient heat exchanger as the surface area presented to the intendedfluid is limited. As a result, a significant amount of the availableheat, imparted by the flame, is lost as hot exhaust gases flow throughthe firetube and up the stack. Thus, conventional U-tube firetubes areexpensive to operate, waste energy used to generate the heat, typicallydo not optimally utilize the heat generated, and release large amountsof waste gas to the environment.

Further, in instances where the process heating requirements change andmore process fluids enter the operation than design load, the onlyalternative has been to replace the equipment with larger units.

Clearly there is a need for improved heat exchangers which are capableof efficiently and cost effectively transferring thermal input to fluidsto be heated.

SUMMARY OF THE INVENTION

Generally, embodiments of firetubes, disclosed herein, have an increasedsurface area without resulting in an overall increase in the size of thefiretube due to a plurality of thermally conducting passageways whichextend through the firetube and direct fluids to be heated therethrough.Each of the passageways has a wall for heat transfer which adds to theexternal surface area of the firetube resulting in the increased surfacearea. In an embodiment, fluids are caused to rise through thepassageways as a result of a temperature differential in the vesselcreating a natural convective circulation or thermosiphon effect, thefluids below the firetube being cooler and more dense and the fluidsabove being warmer and less dense. Embodiments of the firetube aresuitable for use in direct and indirect-fired vessels.

Advantageously, where process fluids comprise emulsions of water andhydrocarbons having different coefficients causing them to expand andcontract at different rates, the expansion and contraction as the fluidenters and leaves the relatively small diameter passageways aids incoalescence of like molecules, which assists in separation of thedifferent constituents a vessel.

In a broad aspect, a firetube is adapted to extend horizontally into avessel for heating fluid therein. The firetube has a gas inlet, a gasoutlet and at least one flowpath therebetween and conducts hot gasesalong the flowpath from the gas inlet to the gas outlet. The firetubecomprises a plurality of passageways, spaced along the flowpath forpassing fluid upwardly therethrough. Each passageway extends generallyupwardly from a fluid inlet at a lower portion to a fluid outlet at anupper portion and has a thermally conductive wall extending through theflowpath for conducting heat from hot gases to the fluid passingtherethrough.

Further, a heat exchanger for a vessel comprises the firetube accordingto embodiments of the invention. The firetube is suitable for use in adirect-fired vessel where the fluid is a process fluid to be heated bythe firetube, the firetube being immersed in the process fluid. Thefiretube is also suitable for use in an indirect-fired vessel where thefluid is a heat transfer fluid to be heated by the firetube. In thiscase, the heat exchanger further comprises a fluid-to-fluid heatexchanger for flowing the process fluid therethrough, the fluid-to-fluidheat exchanger being immersed in the heat transfer fluid. In embodimentsthe heat transfer fluid is glycol.

Embodiments of the firetube are suitable for installing in new vesselsor can be used to retrofit existing vessels. As the size of the expandedsurface area firetube is substantially the same as the existing priorart firetube, it can be simply installed through the existing manway forflanged connection thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate a prior art U-tube firetube, more particularly,

FIG. 1A is a plan view of the U-tube shown installed in an manway in afront wall of a vessel, a major portion of the vessel having beenremoved for clarity;

FIG. 1B is a plan view of a front wall of the vessel according to FIG.1A, illustrating an inlet and an outlet of the U-tube installed in afront wall of the manway; and

FIG. 1C is an elevation view of the front wall of the oval manway ofFIG. 1B illustrating the inlet and the outlet;

FIG. 2A is a plan view of a cross-section of one embodiment of a U-tubefiretube installed in a vessel, a major portion of the vessel havingbeen removed for clarity, the firetube having a dividing wall extendingpartially along the firetube and being fit with a plurality of thermallyconductive passageways;

FIG. 2B is a side cross-sectional view of one thermally conductivepassageway fit to portion of a firetube according to FIG. 2A;

FIG. 2C is a cross-sectional view of a firetube in a direct-fired vesselincorporating an embodiment of the thermally conductive passageways;

FIG. 2D is a cross-sectional view of an firetube in an indirect-firedvessel incorporating an embodiment of the thermally conductivepassageways;

FIG. 3A is a plan view of a cross-section of the U-tube firetube of FIG.2, wherein the dividing wall is formed by a plurality of plates betweena plurality of the thermally conductive passageways;

FIG. 3B is an end cross-sectional view through the firetube of FIG. 3A,along section lines A-A;

FIG. 4 is a plan view of a cross-section of the U-tube firetube of FIG.3A having a first central divider and additional of the passageways withsecond and third dividers for forming two generally U-shaped flowpathsin the body;

FIG. 5 is a perspective view of the firetube according to FIG. 4, thebody being rendered as transparent for greater clarity;

FIG. 6 is a plan view of a cross-section of a U-tube firetube accordingto another embodiment, the thermally conductive passageways forming atortuous flowpath in the body;

FIG. 7 is a plan view of a cross-section of a conduit firetube accordingto another embodiment, suitable for retrofit of a vessel having a priorart firetube according to FIG. 1A;

FIG. 8 is a side, cross-sectional view of an embodiment of the firetubeillustrating a variety of possible profiles for the thermally conductivepassageways;

FIG. 9 is a side, cross-sectional view of the firetube and tubularpassageways according to FIG. 3A;

FIG. 10 is a fanciful illustration of fluid flow through tubularthermally conductive passageways, from fluid inlets below the firetubeto fluid outlets above the firetube; and

FIG. 11 is a fanciful illustration of the fluid flow through thethermally conductive passageways according to FIG. 10 and enhanced bythe action of vortex generators mounted adjacent the passageway inlets.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

As shown in FIGS. 1A-1C, prior art firetubes 10 are generally U-shapedtubes, having a side-by-side gas inlet 12 and gas outlet 14 at a flangedconnection 16 at a front wall 24 of vessel 26. The firetube 10 isgenerally manufactured from round, steel pipe which is welded together,using welded mitres 18 for forming the “U” at an end 20. The prior artfiretube 10 is connected to a manway 22, typically obround in shape toaccommodate the side-by-side inlet 12 and outlet 14. The gas inlet 12connects therethrough to a burner (not shown) for receiving flame andhot exhaust gases therefrom. The outlet 14 connects to an exhaust stack(not shown) for exhausting waste gases therefrom. Flanged connectionsare typically used throughout.

Firetubes, according to embodiments disclosed herein, can beincorporated in new heat exchange vessels or can be used to retrofitexisting vessels to upgrade and enhance the efficiency of heat transfertherein. Heat transfer surface area is increased over conventionalfiretubes by providing a plurality of thermally conductive passagewayswhich extend through the firetube. Fluid in the vessel is heated, notonly from the periphery of the firetube but also through fluid conductedthrough the passageways.

In more detail, and having reference to FIG. 2A-3B, one embodiment of afiretube 30 comprises a hollow shell or body 32 having body walls 38 forcontaining and directing hot gases G therethrough. In use, the firetube30 is fit into vessel V and immersed in a fluid F contained therein. Thebody walls 38 form a portion of the surface area for heat transfer fromthe gas G to the fluid F. Hot gases G circulate through a flowpath 42,from a gas inlet 44 at a first end 46 to a gas outlet 48 at a second end50.

As shown in FIG. 2A, when the firetube's gas inlet 44 and gas outlet 48are located side-by-side, a U-shaped flowpath 42 is formed. As is alsothe case in the prior art, the inlet 44 is adapted for connection to asource of hot gases such as a burner (not shown) and the outlet 48 isadapted for connection to an exhaust stack (not shown). The gas inlet 44and gas outlet 48 are fit to a front or common header wall 34 secured,such as by flanged connection, to the vessel V. The firetube 30 can befit through an obround manway (see FIG. 3B) and is cantilevered orotherwise supported to extend generally horizontally from the front wall36. The firetube 30 has a tube end 36 at a farthest extent from thefront wall 34.

Hot gases G circulate through the flowpath 42, from the gas inlet 44 tothe gas outlet 48, heating the body walls 38 and transferring the heatto fluid F.

The firetube body 32 can be a U-shaped conduit (See FIG. 7) or agenerally open body, the interior of which is then fit with structurefor directing the gases. Various internal gas-directing structure areillustrated in FIGS. 2A, 3A and 4. The gas-directing structure avoidsshort-circuiting of the flowpath 42 and maximizes gas contact with thebody walls 38. With reference to FIG. 2A, the gas-directing structurecan be a first dividing wall 40 extending partially along the hollowbody 32, from a proximal end at the common header wall 34, from alocation between the gas inlet 44 and outlet 48, to a distal end locatedshort of the tube end 36 for forming the generally U-shaped flowpath 42within the body 32. Whether the body 32 is a U-tube conduit (FIG. 7) orfit with one or more dividing walls 40, the surface area can be enhancedby further providing a plurality of thermally conductive passageways 52.

Best seen in FIG. 2B, the passageways are spaced apart along theflowpath 42 and extend through the body 32 from a fluid inlet 62 atlower portion L of the body wall 38 to fluid exit 64 at an upper portionU of the body wall 38. Cooler fluid, to be heated, flows upwardly intothe fluid inlet 62 from below the firetube 30 to exit each passageway 52at the fluid exit 64 above the firetube 30. The passageways 52 have athermally conductive, tubular wall 54, typically formed of the samematerial as the body walls 38, forming an external surface 56 in contactwith hot gases G flowing through the flowpath 42 and an internal surface58 for contacting the fluid F. The walls 54 of the plurality ofpassageways 52 provide additional heat transfer surface over thatconventionally provided by the prior art U-tube firetube.

As shown in FIG. 2C, fluid F circulates from the lower portion L to theupper portion U of the body wall 38 and then back down within the vesselto repeat the cycle. Where no mechanical impetus is provided, the fluidF movement is like a thermosiphon circulation.

Noteably, such an increase in the heat-transferring surface area isaccomplished without an increase in the overall size of the firetube 30.Thus, in an embodiment, the firetube 30 can be installed, as a retrofit,through the obround manway 22 of an existing vessel V, increasing thevessel's heating capability over its original design rating.

In an embodiment, as shown in FIGS. 3A and 3B, a plurality of thethermally conductive passageways 52 can be aligned be integrated withthe dividing wall 40. As shown, the dividing wall 40 is a first wallcentrally located between the inlet 44 and outlet 48. Accordingly, thedividing wall 40 can be formed of a plurality of plates 60,60,60 . . . ,each plate 60 being connected between adjacent passageways 52 fordirecting gases G along the passageways 52 to the tube end 36. Theplates 60 urge gases G from the gas inlet 44 to the dividing wall'sdistal end and back to the gas outlet 48. The plates 60 can be weldedbetween passageways 52. In addition, a plurality of the passageways 52are fit to the firetube 30 along the flowpath 42 for conducting heatfrom hot gases to the fluid passing therethrough.

The number of passageways 52 fit to the flowpath 42 is a function of thedesired or design surface area of the tubular walls 54 while not overlyrestricting the flow of gases G therealong.

In another embodiment, shown in FIGS. 4 and 5, a second dividing wall40B and third dividing wall 40C are provided, forming two, side-by-sideU-shaped flowpaths 42,42. As shown in FIG. 6, a firetube 30 may or maynot have passageways 52 aligned along the first central dividing wall40. The second and third dividing walls 40B, 40C can be connected atdistal ends to more particularly direct the flowpaths. When connected,the second and third dividing walls 40B, 40C form a U-shaped dividingwall 40U wherein a first flowpath 42 is formed from the gas inlet 44,between the first and second divider walls 40,40B, and to the gas outlet48 between the first and third divider walls 40,40C, and a secondflowpath 42 is formed from the gas inlet 44, between the first dividerwall 40 and the body walls 38, and to the gas outlet 48 between thefirst wall 40 and the body walls 38.

Returning to FIG. 6, in an embodiment having a centralized dividing wall40, without passageways 52 integrated therein, a plurality ofnon-aligned passageways 52 are distributed laterally across the flowpath42 to access more of the flow of gas G and increase heat transferrecovered therefrom.

Having reference to FIG. 7, alternatively, a plurality of thermallyconductive passageways 52 can be retrofitted to the otherwiseconventional prior art U-shaped firetube 10 of FIG. 1A.

While the plurality of thermally conductive passageways 52 are used toincrease the effective surface area of the heat exchanger 30, one ofskill in the art would appreciate that too many or too large a diameterof thermally conductive passageways 52 may restrict or interfere withthe circulation of the hot exhaust gases G within the heat exchanger 30.Alternatively, too few thermally conductive passageways 52 may notincrease the surface area sufficiently to increase heat transferefficiency. Further, if the internal diameter of each thermallyconductive passageways 52 is too small for the fluid F, the flow ratethrough the passageways 52 can be ineffective or the passageways couldbecome clogged or plugged by the fluid F or contaminants therein.

In the case of direct-fired systems, shown in FIG. 2C, where thefiretube 30 is immersed in a process fluid F_(P), the passageways 52could be prone to plugging by contaminants entrained within the processfluids F_(P) passing therethrough. For conventional oilfield operations,Applicant believes that each of the passageways 52 could have a diameterin the range of from about 15% to about 18% of the diameter of the gasinlet 44 for achieving effective heat transfer.

In the case of indirect-fired systems, shown in FIG. 2D, the firetube 30is immersed in a substantially clean, heat transfer fluid such asglycol. A fluid-to-fluid heat exchanger 70 is provided for flowing theprocess fluid F_(P) therethrough, the fluid-to-fluid heat exchanger 70being immersed in the heat transfer fluid F. Heat transferred from thegas G to the heat transfer fluid F is transferred the process fluidF_(P). Having minimized risk of clogging of the passageways 52, as cleanfluid F flows therethrough, the passageways 52 could be made with asmaller diameter than in the direct-fired system. Further, in theindirect-fired systems, additional passageways 52 may be added tofurther increase the surface area and thus, increase the heat transferefficiency.

As shown in FIG. 8, in embodiments, the thermally conductive passageways52 can be upright or substantially vertical pipes passing through thebody 32. Having reference to FIG. 9, the thermally conductivepassageways 52 can have a variety of shapes or profiles when viewed incross-section, for example those profiles including those shown viewedfrom left to right, having a narrow fluid inlet 62 with a wide fluidoutlet 64, a wide fluid inlet 62 with a narrow fluid outlet 64, a narrowfluid inlet 62 and exit 64 with an enlarged intermediate portion, andone having a uniform profile from inlet 62 to outlet 64.

With reference to FIGS. 10 and 11, each of the plurality of thermallyconductive passageways 52 has the fluid inlet 62, fluidly communicatingwith the fluid F in the vessel V below the body 32, and the fluid outlet64, fluidly communicating with the fluid F above the body 32. Thearrangement of the fluid inlet 62 and outlet 64 permits the fluid F torise through each passageway 52 and be heated during its passagetherethrough. Applicant believes that the fluid to be heated F iscirculated through the firetube 30 and vessel V as a result of atemperature differential which exists between the cooler fluid F at theinlet 62 and the warmed fluid at the outlet 64. The temperaturedifference would be sufficient to cause a natural convection current ora thermosiphon effect for urging the fluid F to circulate through theplurality of passageways 52 and cause circulation of the fluid Fthroughout the vessel V.

As the fluid F heats, the fluid F becomes less dense and rises withinwithin each of the passageways 52, passing therethrough, receiving heatfrom the tubular wall 54 and rising within the vessel V. The heatedfluid F exits the outlet 64 at a temperature greater than that of thenominal vessel temperatures and releases heat thereto. As heated fluid Ftransfers its heat, the fluid F begins to sink within the vessel Vestablishing a convective circulation.

In an embodiment, as seen in FIG. 11, heat transfer can be enhanced fromthe gas G to the fluid F in the passages 52 by the addition of vortexgenerators 80 adjacent one or more of the passageway fluid inlets 62.The vortex generators 80 impart a swirl of the fluid rising within thepassageway 52. The swirling action acts to increase the retention timeof the fluid F within the thermally conductive passageways 52,permitting more efficient transfer of heat to the fluid F therein.Further, it is believed that the vortex generators 80 cause more cooleror dense fluids, flowing through the passageways 52, to move from thecenter of the flow to the outside, effectively creating a laminar flowadjacent the internal surface 58 which aids the heat transfer.

Further, as the heated fluid F becomes hotter, a natural separation ofconstituents occurs between the dense fluid and less dense fluid. Thisphenomenon is particularly advantageous when the fluid F is an unstableemulsion.

Applicant believes, this is a useful phenomenon in the case of vesselssuch as heater-treaters and free water knock-out vessels, whereseparation of hydrocarbons and water can also occur. Applicant believesthat the effect of the fluid F entering the passageways 52, followed byan expansion of the fluid F leaving the passageways 52, aids in theseparation of the hydrocarbons from water. The constituents of theprocess fluid F_(P), particularly the hydrocarbons and the water, havedifferent viscosities and heat coefficients causing them to expand andcontract at different rates. The expansion and contractions aids incoalescence of like molecules which assists in separation of thedifferent constituents.

In an example, employing embodiments discussed herein, for a processvessel having 2 million British Thermal Unit (BTU) heat exchangercapacity, the surface area may be increased as much as 50% compared to aconventional U-tube which is sized to be installed in the same sizemanway. The increased surface area is directly reflected in theincreased heat which can be transferred to the fluid F in the vessel V.

1. A firetube adapted to extend horizontally into a vessel for heatingfluid therein, the firetube having a gas inlet, a gas outlet and atleast one flowpath therebetween, the firetube conducting hot gases alongthe flowpath from the gas inlet to the gas outlet, the firetubecomprising: a plurality of passageways spaced along the flowpath forpassing fluid upwardly therethrough, each passageway extending generallyupwardly from a fluid inlet at a lower portion of the firetube to afluid outlet at an upper portion of the firetube and having a thermallyconductive wall extending through the flowpath for conducting heat fromthe hot gases to the fluid passing therethrough.
 2. The firetube ofclaim 1 wherein the flowpath is generally U-shaped from the gas inlet tothe gas outlet.
 3. The firetube of claim 2 wherein the generallyU-shaped flowpath comprises a U-tube conduit having the gas inletadjacent the gas outlet at a common header wall, the plurality ofpassageways being spaced along the U-tube conduit.
 4. The firetube ofclaim 2 wherein the firetube is a hollow body having body walls, the gasinlet being adjacent the gas outlet at a common header wall, thegenerally U-shaped flowpath comprising: at least one dividing wallextending from between the gas inlet and the gas outlet, partially alongthe hollow body from the common header wall and toward a tube end,wherein the gases are directed to flow along the U-shaped flowpath fromthe gas inlet, about a distal end of the at least one dividing wall, andto the gas outlet.
 5. The firetube of claim 2 wherein at least some ofthe plurality passageways are distributed laterally across the flowpath.6. The firetube of claim 4 wherein at least some of the plurality ofpassageways are integral with the at least one dividing wall.
 7. Thefiretube of claim 6 wherein passageways integral with the at least onedividing wall are substantially aligned and connected therebetween byplates to urge gas along the U-shaped flowpath.
 8. The firetube of claim1 wherein the flowpath comprises two side-by-side flowpaths.
 9. Thefiretube of claim 8 wherein the firetube is a hollow body havingenclosing body walls, the gas inlet being adjacent the gas outlet at acommon header wall, the two, side-by-side flowpaths comprise: a firstdividing wall, intermediate the gas inlet and the gas outlet, andextending from the common header wall toward a tube end,; a seconddividing wall extending from the common header wall intermediate the gasinlet; and a third dividing wall extending from the common header wallintermediate the gas outlet, wherein gases flow along the two,side-by-side flowpaths from the gas inlet, about distal ends of thefirst, second and third dividing walls and to the gas outlet,
 10. Thefiretube of claim 9 wherein: the distal end of the second dividing walland the distal end of the third dividing wall art are connected; andwherein a first flowpath of the side-by-side flowpaths is formed fromthe gas inlet, between the first and second divider walls, and to thegas outlet between the first and third divider walls, and a secondflowpath of the side-by-side flowpaths is formed from the gas inlet,between the first divider wall and the body walls, and to the gas outletbetween the first divider wall and the body walls.
 11. The firetube ofclaim 1 wherein the thermally conductive passageways are pipes extendingsubstantially vertically through the firetube.
 12. The firetube of claim1 further comprising a vortex generator at one or more of the passagewayfluid inlets.
 13. A heat exchanger for a vessel comprising the firetubeof claim 1, wherein the vessel is a direct-fired vessel and the fluid isa process fluid to be heated by the firetube, the firetube beingimmersed in the process fluid.
 14. The heat exchanger of claim 13wherein each of the plurality of passageways has a diameter from about15% to about 18% of a diameter of the inlet.
 15. A heat exchanger for avessel comprising the firetube of claim 1, wherein the vessel is anindirect-fired vessel and the fluid is a heat transfer fluid to beheated by the firetube, the heat exchanger further comprising afluid-to-fluid heat exchanger for flowing a process fluid therethrough,the fluid-to-fluid heat exchanger being immersed in the heat transferfluid.
 16. The heat exchanger of claim 15 wherein the heat transferfluid is glycol.
 17. The heat exchanger of claim 13 wherein the firetubeis obround in cross-section and is installed through an obround manwayformed in the vessel.